Summary
- Over 500 weed biotypes (in the world) have developed resistance to specific herbicides.
- Due to the repeated use of specific herbicides, herbicide-resistant weeds are found in field crops.
- It would be unusual for herbicide-resistant weeds to be present in home lawns or landscapes.
- Various weed control measures reduce the potential for developing herbicide-resistant weeds. These measures include hand weeding, replenishing mulch to suppress weeds, and not repeatedly using the same herbicides.
Herbicide-resistant weeds are not usually problems in home landscapes or gardens; however, due to the abundance of herbicide-resistant weeds, these troublesome plants could easily be introduced into lawn and landscaped areas.
Herbicide resistance was identified more than 60 years ago. Starting in the mid-1990s, the widespread use of herbicide-tolerant crops increased the frequency of weeds becoming resistant to common herbicides, with now over 250 species of weeds having developed resistance to 160 different herbicides that span almost all the herbicide mechanisms of action (how herbicides kill weeds). Resistant weeds are found in 66 countries worldwide, making herbicide resistance a global problem.
Herbicide resistance is the acquired ability of certain weeds to survive an herbicide application that was previously known to control the weed. This ability is present naturally within small plant populations, but the repeated use of the same herbicide group allows these genetically different plants to persist and reproduce, becoming a larger percentage of the future plant population. Over 35 different weeds have developed resistance to the herbicide glyphosate (Roundup-type products). Glyphosate is a non-selective herbicide that is routinely used to control weeds in landscapes and other areas.
Repeated use of herbicides with the same mode of action can result in the increase of herbicide-resistant plant populations.
Adapted from Characteristics of Herbicides for Turf, Ornamental Landscapes, and Aquatics – 3rd Edition by B. McCarty, M. Cutulle, and A. Gore, 2021.
Many homeowners may think that if the weeds were not killed when they were sprayed, they must be resistant to that herbicide, forgetting that many factors determine whether herbicides effectively control weeds. Factors include selecting the right herbicide, using and applying the appropriate rate, and the timing of the application. The size of the weeds and environmental factors, such as drought or rain, near the time of spraying can also impact how well an herbicide works.
Some weeds are classified as tolerant to certain herbicides, meaning the herbicide never controlled the weeds and are usually not listed for control on the herbicide label. Herbicide labels give instructions on how best to use herbicides. It is important and required by law, to carefully read the herbicide label as it states precautions, application mixing rates, and a list of weeds controlled. The label will also list tolerant landscape plants that the herbicide can be used around. For more information on pesticide labels and safety, please see HGIC 2751, Pesticide Safety.
The annual grasses goosegrass (Eleusine indica (L.) Gaertn.) and annual bluegrass (Poa annua L.) are examples of weeds with herbicide-resistant biotypes. Herbicide-resistant biotypes are not visually different than susceptible ones. They are generally less aggressive but produce seed that emerges into herbicide-resistant seedlings. One resistant plant reproducing by seed can quickly develop into a resistant population.
Goosegrass (left) a common lawn and landscape weedy grass with herbicide-resistant biotypes.
Ted Whitwell, ©2023, Clemson Extension
Annual bluegrass (right) also a common lawn and landscape weedy grasses with herbicide-resistant biotypes.
Barbara Smith, ©2023, Clemson Extension
Herbicide-resistant weeds, such as annual bluegrass and goosegrass, have developed in turf areas. In 2019, a national survey of landscape and turfgrass managers indicated that herbicide-resistant weeds were a problem, and they observed weeds resistant to glyphosate and triazine herbicides. Based on this survey and their experience, the survey concluded that widespread herbicide-resistant weed problems were not currently a major landscape concern. The use of non-chemical weed control measures, including mulches and hand weeding, along with adopting a practice of rotating herbicides with different mechanisms of action, are important in preventing the development of herbicide-resistant weeds.
How to Avoid Herbicide-Resistant Weeds?
- Use weed-free mulch.
- Clean equipment after use to prevent introducing weed seeds into new areas.
- Use diverse non-herbicide weed control measures such as hand weeding and mulching to suppress landscape bed weeds.
- Avoid repeated applications of the same herbicide. Use herbicides with different modes of action.
References:
- Anderson, W.P. 1996. Weed Science Principles and Applications (3rd Ed.). West Publishing, Minneapolis, ME.
- Derr J.F., Neal J.C., and P.C. Bhowmik. 2020. Herbicide resistance in the nursery crop production and landscape maintenance industries. Weed Technology; 34:437-446.
- Heap, I. The International Survey of Herbicide Resistant Weeds. https://www.weedscience.org/Home.aspx. Assessed June 27, 2023
- McCarty, B., Cutulle, B., and A. Gore. 2021. Characteristics of Herbicides for Turf, Ornamental Landscapes, and Aquatics (3rd Ed.). KDP Publishing
Originally published 8/23
